The performance of wind turbines is affected by turbulent wind conditions that cause structural loads on the wind turbine and its components. Currently, structural loads may be reduced by measuring incoming wind speed and wind turbulence in front of the wind turbine with remote wind sensing apparatuses. The data gathered by the wind sensing apparatuses may be used by feed-forward wind turbine controllers to proactively compensate for wind velocity and direction changes prior to impingement of the air flow on the wind turbine e.g. by adjusting the yaw of the nacelle and pitch angle of the blades to protect components and maximize the performance of the wind turbine.
A number of optical and acoustic methods allow remote measurement of wind speed in the atmosphere. These include the technique of light detecting and ranging (LIDAR), also referred to as coherent laser radar (CLR) and coherent Doppler LIDAR (CDL). LIDAR involves the emission of a coherent light beam and detection of the weak return reflected or scattered from a distant target. The technique provides a way to measure the line-of-sight component of wind speed via detection of the Doppler shift for light backscattered from natural aerosols (particles of dust, pollen, droplets, etc.) in the atmosphere.
Sonic detecting and ranging (SODAR) is another commonly used Doppler-based method of remote atmospheric wind profiling. It involves the emission of sound pulses and relies on the detection of the weak echo scattered from temperature and velocity fluctuations in the atmosphere. It measures the wind velocity via the Doppler shift of the acoustic pulses in a manner analogous to LIDAR.
Several LIDAR or SODAR mounting positioning options in wind turbines are currently known. A LIDAR may be mounted on the nacelle facing the incoming wind. However, the restricted field of view caused by the rotating blades of the turbine results in only approximately 75% of measurements being successful, with the laser beam striking a blade in the remaining 25% of cases. For the same reason, it takes longer to gather measurements. It is also known that a rearward-facing LIDAR may be mounted on the nacelle on a pan-and-tilt scanner to measure wake wind speed deficit and wander.
Mounting the LIDAR on the hub overcomes the restricted field of view of nacelle mounted LIDAR systems, however its installation and maintenance is difficult due the height of the tower and the rotation of the hub.
The accuracy of the measurements taken by remote wind sensing apparatuses mounted on the nacelle or on the hub is furthermore affected by the bending moments and swaying experienced by the wind tower in strong winds.
SODARs are less likely to be mounted on the nacelle or the hub because the noises generated at the nacelle and/or at the hub interfere with the SODAR's sonic signals.
A known approach to overcome the known drawbacks of mounting the LIDAR or a SODAR on the nacelle or the hub is to position the LIDAR or the SODAR on the ground some distance ahead of the wind turbine and directed upwardly to measure wind speed and wind fluctuations of the wind in front of the wind turbine. Although this solution is easily implemented inland, it is very costly to implement offshore as it would require building a separate offshore platform to carry the remote wind sensing apparatus.
In general, it is known that the turbulent movements in the wind will evolve between the time they are measured and when they reach the turbine, causing errors in the preview wind measurements. In addition, the rotor blades have the effect of slowing down the mean velocity of the incoming wind near the rotor and further altering the turbulence characteristics. A full and detailed interpretation of these events is difficult since the wind field is currently being probed only along a single line, and hence no information can be obtained on the transverse structure of the gusts.
There therefore is a need to further improve wind measurement systems for wind turbines.